How To Get Permission For Licensed Characters?

Sorry if this has already been addressed somewhere -- I'm almost certain it has, but for some reason the search I'm running is coming up with nothing except random topics that have no relation whatsoever...

I'm needing to make a "Mario Brothers" cake. I've responded to the person that I can't make licensed images, even though of course everywhere you look online you see professional bakers...making licensed images out of fondant or with buttercreme image transfers...so if she wants the cake from me and she wants the characters, she needs to buy a set to put on the cake that I deliver.

But I'm just wondering - does someone here already have information about how to write to ask for permission? I'd like to leave open that possibility for someone willing to pay for my trouble to do that. I'm not sure I'd even attempt to ask Disney, but I'm wondering if Nintendo might be more accomodating.

Thanks for any guidance you can give. Or if you know of a topic where this has already been discussed, I'd love to have the direct link.